Thermionic energy converter



United States Patent 3,515,908 THERMIONIC ENERGY CONVERTER French Caldwell, 1507 N. Brookwood Drive, Pascagoula, Miss. 39567 Filed Sept. 14, 1966, Ser. No. 579,257 Int. Cl. H01j 45 00 U.S. Cl. 310-4 9 Claims ABSTRACT 0F THE DISCLOSURE A thermionic energy converter comprises supports for the anode and the cathode, the supports being of electrical insulating material and being hermetically sealed together about the margins of the anode support remote from the heated surface of the cathode support so as to cool the hermetic seal. The cathode support electrically insulates the seal from the cathode, and preferably the anode support insulates the seal also from the anode.

This invention relates to thermionic energy converters, in which the How of electrons from a relatively hot electron-emissive cathode to a relatively cold electron-gathering anode is utilized as a means for converting heat into electrical power.

This invention is an improvement on the device disclosed in Pat. No. 2,759,112 (Reissue Pat. 24,879), granted to my late father, Winston Caldwell, which is the basic and pioneer patent relating to these power sources.

Since the advent of my fathers invention, referred to above, many proposals for practicing my fathers invention commercially have been advanced. Principally, these have taken two of the main forms advanced by my father: In the first, the anodes and cathodes are spaced apart by a high vacuum; and in the second, the vacuum space between the anodes and the cathodes contains a small quantity of vapor at very low pressure. For example, cesium vapor at l mm. of mercury may be introduced into the vacuum space to coat the anode and cathode, thus controlling their respective electron work functions, and at the same time reducing the space charge in the space between the cathode and anode.

Regardless of which of the many potential forms the apparatus takes, it is still necessary to provide a vacuum in the space between the anode and the cathode, the anode and the cathode having desirably uniformly spaced confronting areas.

A problem therefore arises when it is attempted to evacuate and seal converters of the type of my fathers invention. There are several known ways of doing this. In the first, the hermetically sealed envelope is of glass or other material that can be readily deformed under heat. In such a device, the anode or the cathode is disposed within the glass envelope and the other of the anode and cathode is supported by the envelope itself. The glass envelope is then evacuated and heat sealed. An example of this type of construction is seen in Bloss Pat. No. 3,217,- 189, Nov. 9, 1965.

A difficulty arises in trying to use a glass envelope, in that the deformation of the glass under heat to seal the envelope makes it impossible to space the anode and the cathode from each other with the necessary accuracy. Devices of this type need anodes and cathodes of extended surface closely spaced apart, and it is important to maintain the spacing both close and uniform. Therefore, glass 3,515,908 Patented June 2, 1970 ICC envelope devices that are sealed by deformation are unsuitable. Instead, it is preferable to use rigid envelopes which can be assembled without deformation.

yExamples of rigid envelope constructions are seen in Beggs Pat. No. 3,176,164, Mar. 30, 1965, and Clement Pat. No. 3,211,930, Oct. 12, 1965. Each of these patents discloses converters having rigid envelopes on and within which are disposed ceramic materials for insulating and supporting purposes.

One of the disadvantages of rigid envelope-type converters, and particularly rigid envelope-type converters that include metal and ceramic components arranged as in the prior art, is that it is very difcult to produce an effective high temperature hermetic seal between the ceramic and metal parts. The emitters of these devices operate at relatively high temperature, of the order of 1500i C., more or less, and the metal and the ceramic components have distinctively different coefficients of thermal expansion. In constructing multiple diode devices of this type, a plurality of ceramic-to-metai seals is required, each of which is critical to the hermetic seal. Indeed, the inability to provide elfective hermetic seals in devices of this type has rendered such devices commercially impractical prior to the present invention.

Accordingly, it is an object of the present invention to provide thermionic energy converters, in which an effective hermetic seal can be produced and maintained despite high temperature operation and with no deformation of the parts.

A further object of the invention is the provision of multiple diode converters having only one primary hermetic seal.

Another object of the present invention is the provision of such converters, in which means are provided for regulating the temperature of different portions of the device.

Finally, it is an object of the present invention to provide converters of this type, which will be relatively simple and inexpensive to manufacture, assemble and install, easy to maintain and operate, and rugged and durable in use.

Other objects and advantages of the present invention Will become apparent from a consideration of the following description, taken in connection with the accompanying drawing, in which:

FIG. 1 is an elevational cross-sectional View of one embodiment of a converter according to the present invention;

FIG. 2 is a fragment of a view similar to FIG. 1 but showing a second embodiment of the invention;

FIG. 3 is a view similar to FIG. 2 but showing still another embodiment of the invention;

IFIG. 4 is an enuarged fragment of a portion of FIG. 1, showing still another modification;

FIG. 5 is a view similar to FIGS. 2 and 3 but showing an annular embodiment of the invention; and

FIG. 6 is a cross-sectional view taken on the line 6 6 of FIG. 5.

Referring now to the drawing io greater detail, and first to the embodiment of FIG. 1, there is shown a thermionic energy converter comprising a rst rigid rectangular support 1 of electrically insulating material. Any electrically insulating material which is resistant to high temperatures, say, in the range of about 1200-1700 C., may be used. Ceramics are preferred, and among the many suitable ceramics is alumina. Beryllia and yttria are also quite suitable.

Support 1 comprises a flat plate 3 having upstanding integral marginal flanges 5 on all four sides. Plate 3 has an exposed surface 7 by which heat is applied to the converter by radiative, Conductive or convective heating. A plurality of cathodes 9 are supported on the inner side of plate 3 in spaced relationship from each other. Cathodes 9 in the illustrated embodiment are thin electrically conductive at rectangular plates. Their exposed surfaces are of electron-emissive material, a number of which materials are known in this art. For example, the cathodes themselves can be of an emissive material such as molybdenum, tungsten or rhenium. Or a base metal cathode can be coated with an emissive substance such as calciu-m oxide, strontium oxide, barium oxide, etc. or mixtures thereof. Also useful as materials for the cathode-emissive surface are the metalloids such as the borides, carbides, nitrides, etc. Uranium carbide is a particularly suitable metalloid.

The converter also comprises a second rigid support 11 of electrically insulating material, which may be the same material as support 1, and which comprises a generally flat rectangular plate 13 parallel to but spaced Ifrom plate 3. Plate 13 is bounded by four side edges 15 disposed in planes perpendicular to the general plane of plate 13. Support 11 has an exposed surface 17 by which heat may be extracted from the device, by radiative, convective or conductive cooling.

Plate 13 includes a plurality of generally rectangular pedestals 19 that upstand from plate 13 and extend toward first support 1. Pedestals 19 are spaced apart between themselves, and between themselves and the flanges 5 of support 1, =by recesses 21 that extend in a direction away from first support 1, that is, that extend away from the heated surface 7 into adjacency to the cooled surface 17. If desired, a vapor of an alkali metal such as cesium, or other special atmosphere, may be provided in the interior of the device, at a suitably low absolute pressure such as 1 mm. of mercury. Recesses 21 can thus serve as reservoirs for gaseous cesium, and cesium pressure can be controlled by controlling the temperature of surface 17. A controlled temperature gradient between the anodes and surface 17 can in turn be established by the design and construction of pedestals 19.

Mounted on each pedestal 19 is a thin flat rectangular plate in the for-m of an anode 23. Each anode 23 is parallel to and spaced a short distance from a corresponding cathode 9, and is offset in its plane so that it does not exactly overlie the corresponding cathode 9. The ends of alternate anodes and cathodes overlie, however, and are electrically interconnected by means of electrically conductive spacers 25 of metal or the like, by which the cathodes and anodes are interconnected in series and also the desired spacing between them is maintained.

Anodes 23 may be of the same material as cathodes 9, because they are at substantially lower temperature than the cathodes, for example, about 600 C. In any event, they are of electrically conductive material and, in addition to the recited cathode materials, may be made of any of a number of materials suitable for use at the relatively lower anode temperatures, such as silver, copper, etc., and would preferably be coated with or contain a material to maintain a relatively low anode electron work function.

A cathode lead 27 is provided in the form of a thin- Walled metal tube in electrical connection with one of the cathodes 9. Lead 27 has lholes 29 therethrough for the evacuation of the space between the rigid supports 1 and 11, and also for the introduction of a gas such as cesium desired. Lead 27 extends through and is hermetically sealed with support 11.

An anode lead 31 is also provided, in the Iform of a thin-walled hollow tube that is interconnected with the anode 23 at the opposite end of the series from cathode lead 27. Lead 31 passes through and is hermetically sealed to second rigid support 11. The inner end of lead 31 can be closed by the associated anode 23, and in any case the outer end is closed and sealed and is used only as an electrical lead.

First and second rigid supports 1 and 11 are bonded together about edges 1S and about the inner edges of flanges 5, by means of a hermetic seal 33. Seal 33 may be formed by metallizing together with soldering or brazing, as by the deposit on the separated surfaces of a refractory material such as molybdenum or tungsten or the like, followed by high temperature bonding; or seal 33 may be formed by other means of sealing ceramics known to the art. Hermetic seal 33 thus extends continuously about and seals the joint between supports 1 and 11, so that upon evacuation, the insulating material of supports 1 and 11 forms a hermetic envelope entirely about the anodes and cathodes, and also a mechanical support for the anodes and cathodes, and also the thermally conductive media for imparting heat to t'he emitting cathodes and cooling the collecting anodes.

It is particularly to be noted that seal 33 is located in the coolest portion of the converter, most closely adjacent the exposed surface 17 from which heat leaves the converter. In this way, the problems heretofore associated with forming and maintaining a hermetic seal, by virtue of the high temperatures to which the seal is exposed in practice, are mitigated to the point that they are no longer significant.

It is also to be observed that a construction is provided in which the temperature gradient, from highest temperature adjacent exposed surface 7 to lowest temperature adjacent exposed surface 17, can be utilized to maintain the different portions of the converter at the respective optimum temperatures. For instance, cathodes 9 require a desirably high electron-emissive temperature and are accordingly closest to surface 7. Anodes 23, however, ree quire a distinctively lower temperature `and are accordingly farther away from surface 7. Moreover, the height of Pedestals 19 can be regulated so as to regulate the temperature of anodes 23. Furthermore, the recesses 21 between and at both ends of the pedestals 19 may be extended to such a depth adjacent relatively cool surface 17 as to provide reservoirs for cesium or the like of desirably lower temperature, thereby to regulate the vapor pressure within the converter without the need for maintaining the converter in fluid communication with an external cesium reservoir or the like. The lead 27 through which the converter is evacuated or through which gas is introduced into the evacuated converter can accordingly be sealed and used thereafter only as an electrical lead. Finally, it is to be noted that the hermetic seals at 33 and between the material of support 11 and the leads 27 Iand 31 are all on the relatively cool side of the converter.

A modified form of the present invention is shown in FIG. 2, which is similar to the embodiment of FIG. 1 except that the first rigid support 35 has no flanges. Instead, the converter is provided with metal cladding 37, which may for example .be of stainless steel or a refractory metal, and which comprises a plate 39 coextensive with and on the hot side of first support 35, there being flanges 41 about all four marginal edges of plate 39 and preferably formed integrally with plate 39 so that there is no problem of providing a hermetic seal between plate 39 and flanges 41. Instead, flanges 41 are hermetically sealed to second rigid support 11 by means of a hermetic seal 33', which may for example be of the metal-to-ceramic type. Flanges 41 are preferably made as thin as practical, in order to minimize heat transfer to seal 33.

Still another form of the invention is shown in FIG. 3, which is like FIG. 1 except that metal cladding 43 is provided, comprising a plate 45 on the hot side of the first rigid support, sides 47 upstanding from the edges of plate 45, and a rim 49 that extends over the marginal edges of the cold side of the converter. As in FIG. 1, however, the hermetic seal is between the lirst and second rigid supports, and may for example be the same ceramic-to ceramic seal as in FIG. 1. Therefore, metal cladding 43 need perform no seal function at all.

It will therefore be apparent, from a consideration of FIGS. 2 and 3, that metal cladding or other metal covering may be provided, to perform a support or protective function for the converter. As shown by FIG. 2, the metal cladding can if desired function as part of the hermetic seal. But as shown by FIG. 3, it is not necessary that the metal cladding participate in the sealing function. In any event, the hermetic seal will remain in the cold portion of the converter.

FIG. 4 shows another embodiment, in which a first rigid support 51 supports spaced cathodes 53 in a form of metalized areas. The technique of metalizing the surfaces of electrical insulating materials such as ceramics is well known and need not be described here. Suffice it to say that metalized deposits of refractory metals such as molybdenum or tungsten or rhenium can be easily applied by known techniques, to any desired depth. It is thus unnecessaryto provide cathodes 53 in the form of separate plates or foils.

A second rigid support`55, of electrical insulating material like support 51, comprises a plurality of pedestals 57 as in the case of FIG. l; but in contrast to FIG. 1, there are no separate spacers but rather integral spacers 59 that are integral with pedestals 57 and that extend toward and contact first rigid support 51 to maintain supports 51 and 55 the desired distance apart. Anodes 61 are provided on pedestals 57 in the form of metalized areas as in the case of cathodes 53; and these anodes 6-1 are electrically connected with cathodes 53 in the same series arrangement as in FIG. 1, by the expedient of extending the surface of anodes 61 about two sides of the integral spacers 59 so that the anodes 61 contact the cathodes 53.

The series relationship of the anodes and cathodes of the various embodiments has been several times mentioned. It will be understood, however, that the cathodes and anodes may instead be arranged in parallel if desired. Also, a single diode, comprising one cathode and one anode, can be constructed so as to embody the present invention.

FIG. 5 shows another embodiment of the invention. Heretofore, the embodiments described have been essentially planar or flat. FIG. 5, by contrast, is a cylindrical or annular embodiment. In the embodiment of FIG. 5, there is provided a first rigid support 63 comprising an annular or cylindrical half sleeve 65. There are a pair of half sleeves 65, disposed in confronting relationship as seen in FIG. 6, so as to complete an annular or cylindrical construction.

At each of its ends, each half sleeve 65 has a generally radially outwardly extending ange 67 that has Ian reception of a corresponding annular projection (not shown) at the other end of the device. These projections fit into the annular recesses 69, so that a plurality of cylindrical units can be assembled in end-to-end relationship. If desired hermetic seals as previously described or other seals can be used to secure these units together in this end-toend relationship, the seals being located at the radially outermost, that is, the coolest, parts of the assembly.

Each half sleeve 65 has an exposed inner annular surface 71, in the form of a half cylinder. It is to` this surface that the heat is applied.

The converter of FIGS. 5 and 6 also comprises a pair of second rigid supports 73, of generally part-cylindrical shape. Each support 73 comprises an outer sleeve segment 75 generally concentric with sleeves 65. Each segment 75 has annular edges 77 at opposite ends thereof, which are disposed in its radially outermost portions and which lie in radial planes perpendicular to the axis of the unit. A hermetic seal 79, as described above, is located between the radially outer portions of flanges 67 and the annular edges 77. The radially outermost surfaces of sleeve segments 75 and anges 67 define an ex- 6 posed surface 81, by which heat is transferred from the converter.

As will be seen in FIG. 6, the converter is bisymmetrical on opposite sides of the plane which is defined by the joint 83 between the two halves of the converter. This joint 83 can if desired be hermetically sealed in its radially outermost portions; but as the joint 83 is independent of the evacuated space between the anodes and the cathodes, such a seal would be provided only if, say, it were necessary to prevent the escape of heated gases from the central passageway 85 which is defined between the two converter halves.

It willbe understood that heat may be applied to converters according to the present invention in any of the conventional manners. Thus, heat may be applied in any of the usual ways, as from hot gases, hot liquids including molten metals, or pyrogenic solid systems such as nuclear fuels. Similarly, heat may be removed from the cathode surface by transfers to or through vapor, liquid or solid phase media, or by radiation through a vacuum. Similarly, when speaking of the relatively hot or relatively cold surfaces of the converter as being exposed, it will be understood that the exposure referred to is exposure to a heating or cooling heat transfer medium or zone, and not necessarily exposure in the sense of exposure to the ambient atmosphere.

From a consideration of the foregoing disclosure, therefore, it will be evident that all of the initially recited objects of the present invention have been achieved.

Although the present invention has been described and illustrated in connction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit of the invention, as those skilled in this art will readily understand. Such modifications and variations are considered to be within the purview and scope of the present invention as defined by the appended claims.

Having described my invention, I claim:

1. A thermionic energy converter comprising at least one electron-emissive heatable cathode and at least one electron-collective coolable anode confronting but spaced from each other, first rigid support means of electrical insu-lating material supporting the cathode, said rst support means having a surface by which heat may be applied to the converter, second rigid support means of electrical insulating material supporting the anode, said second support means having a surface by which heat may be extracted from the converter, means defining a hermetic seal that seals the space between the anode and the cathode, said first support means insulating said cathtode from said seal, said second support means insulating said anode from said seal, and electrically conductive leads from said cathode and said anode, said space between said anode and said cathode being at Subatmospheric pressure.

2. A converter as claimed in claim 1, said seal sealing with portions of rigid electrical insulating material integral with said first support means.

3. A converter as claimed in claim 1, said first support means having integral flanges thereon that extend away from the first support means on the same side as the anode, said seal sealing with said anges.

4. A converter as claimed in claim 1, said first and second support means completely enclosing said anode and cathode.

5. A converter as claimed in claim 1, said leads passing through said second support means in hermetically sealed relationship.

`6. A converter as claimed in claim 1, at least one of said leads comprising a thin wall hollow tube whose interior communicates with the space between the anode and cathode.

7. A converter as claimed in claim 6, both of said leads comprising thin wall hollow tubes.

7 8. A converter as claimed in claim 1, there being a seal being in the form of a at ribbon perpendicular to plurality of said cathodes and a plurality of said anodes adjacent portions of said cathode. confronting but spaced from corresponding cathodes, and means electrically interconnecting one member of References Cited each anode-cathode pair with a member of an adjacent 5 UNITED STATES PATENTS anode-cathode pair, said first support means electrically insulating all said cathodes from said seal, said second support means electrically insulating all said anodes from Said Seal. MILTON O.HIRSHFIELD, Prlmary Examlner 9. A converter as claimed in claim 1, said hermetic 10 D. F. DUGGAN, Assistant Examiner 3,426,220 2/1969 Block et al 310-4 

